In liquid chromatography, when using separation columns with diameters smaller than approximately 2 mm, connecting capillaries having an inner diameter of 50-100 micrometers are required. Such connecting capillaries are, for example, arranged between the sample injector of the liquid chromatograph and the separation column, or between the exit of the separation column and the detector. The connecting capillaries have to have the mentioned small inner diameters in order to maintain the separation performance of the column.
When using separation columns with larger inner diameters one usually employs steel capillaries as connecting capillaries. With the previously mentioned small diameters, however, steel capillaries can no longer be used, since their inner roughness and the non-constant diameter quickly lead to clogging. It is known to use capillaries made of fused silica for the mentioned small diameters. Fused silica capillaries are also used in gas chromatography or capillary electrophoresis as separation columns. These fused silica capillaries can be manufactured with constant diameter and smooth inner surface in the mentioned diameters of approximately 50 to 100 micrometers.
Connecting capillaries of fused silica for high-pressure liquid chromatography with inner diameters of 50 or 100 micrometers and with an outer diameter of 1/16 inch (1.6 mm) and with the corresponding 1/16 inch fitting are commercially available. The known connecting capillaries are coated on their outside with aluminum, and above this is a coating of fluorinated polymers, for example PVDF. Such connecting capillaries, however, have proved in practical tests not to be satisfactory. On the one hand, the coat of PVDF tends to irreversibly withdraw, thus leaving a dead volume in the fitting region, which deforms the peak form of the chromatographic peak and thus substantially reduces the separation performance of the column. On the other hand, the capillary is pushed out of the fitting during extended operation and when using conventional fittings.
In view of the prior art it is an object of the invention to provide a connecting capillary for analytical measurment technology which has a small and substantially constant inner diameter, a smooth inner wall, and which can be equipped with a fitting in an easy way, and which does not have the previously mentioned disadvantages. In particular, it is to be avoided by the invention that additional dead volume is introduced in the fitting region, even if the fitting is in use for a longer period.